How many different standard amino acids are commonly found in the proteins of living organisms?

Introduction / Context:
Proteins are essential biomolecules that perform a wide variety of functions in living organisms, including structural support, catalysis, transport, and regulation. All proteins are built from smaller building blocks called amino acids. While many different amino acid like compounds exist in nature, only a specific set of amino acids is routinely used in the genetic code to build proteins. This question asks for the number of these standard amino acids.

Given Data / Assumptions:

• The focus is on standard amino acids that are encoded by the genetic code.
• These amino acids are the ones commonly found in proteins across most forms of life.
• Options range from very large numbers like 1500 and 3000 to much smaller numbers.
• We assume typical textbook classification of standard amino acids.

Concept / Approach:
The universal genetic code uses codons, which are triplets of bases in mRNA, to specify amino acids during protein synthesis. There are 64 possible codons, of which 61 code for amino acids and 3 act as stop signals. Despite the number of codons, they code for 20 standard amino acids due to redundancy in the code. These 20 include examples such as glycine, alanine, valine, lysine, and tryptophan. Some organisms and special situations may involve additional rare amino acids such as selenocysteine, but in basic biology and most exam contexts, the accepted answer is that proteins are built from 20 standard amino acids.

Step-by-Step Solution:
Step 1: Recall that the genetic code translates sequences of three nucleotides into amino acids. Step 2: Understand that although 64 codons exist, they specify a smaller set of standard amino acids because many codons are synonymous. Step 3: Remember the widely taught fact that there are 20 standard amino acids in proteins. Step 4: Compare this number to the options given and note that only 20 matches the standard figure. Step 5: Recognise that values like 1500 or 3000 refer more to the number of proteins or possible combinations, not to distinct amino acid types. Step 6: Values like 4 and 64 refer to nucleotide bases and codons, not the number of different amino acids.

Verification / Alternative check:
Molecular biology and biochemistry textbooks consistently list 20 standard amino acids, often grouped into categories such as non polar, polar, acidic, and basic. They also provide codon tables showing which codons correspond to which amino acids. The same sources mention that life uses these 20 as the basic alphabet of protein structure. Occasional mention is made of additional amino acids like selenocysteine, but exam questions about the standard count overwhelmingly expect the answer 20.

Why Other Options Are Wrong:
1500 and 3000: These numbers are far too high and more likely to describe numbers of proteins or genes rather than amino acid types. 4: This number corresponds to the four main nucleotide bases in DNA and RNA, not to amino acids. 64: This is the number of possible codons in the genetic code, not the number of amino acids.

Common Pitfalls:
Students sometimes confuse different biological numbers, such as the number of codons, bases, or amino acids. Another mistake is to overthink the question and consider rare amino acids. For most general knowledge and exam questions, remembering that there are 20 standard amino acids in proteins is sufficient to answer correctly.

Final Answer:
The correct answer is 20.